Whois their IP and grab their Host/ISP. Contact their abuse address describing the IP theft. ISPs need to have well defined processes to deal with these complaints or they risk damaging their upstream relationships.
It doesn't work for real valuable IP because we don't have Internet court and even in obvious cases only real court can tell who is 'IP theft'. So creators have to spend a lot of time/money to prove obvious things while the scammer can change providers easily to DMCA resistant one.